Controlling the operation of a hydraulic component in a hydraulic circuit, such as a piston and cylinder assembly, can be accomplished using an electrically controlled hydraulic valve. An electrically controlled hydraulic valve typically includes an inlet port and an outlet port for fluid flow, a displaceable valve element, which may allow or block fluid flow through the inlet port, and a solenoid having an armature associated with the displaceable valve element. When current is applied to the solenoid, the armature will move under electro-magnetic forces generated by the solenoid. The moving armature can cause the attached valve element to move for a certain displacement responsive to the current level. The displacement of the valve element is generally a linear or non-linear function of the amount of current applied to the solenoid.
Typically, when the current applied to the solenoid is increased from zero to a certain level, the valve element is displaced from an origin position to a position for a certain displacement corresponding to the current level. For example, when current I0 is applied to the valve, the valve element may be displaced for displacement X0 from the origin position to a position where the valve just opens enough to allow fluid flow through the valve inlet port. In a displacement-current relationship map, the associated current level I0 and displacement X0 of the valve element are typically referred to as coordinates of a “cracking point.” Therefore the cracking point is associated with a cracking point current I0 and a cracking point displacement X0 of the valve element. Before a valve is used for hydraulic control, the cracking point may need to be determined. Then the cracking point current I0 can be added to subsequent solenoid current commands to compensate for the corresponding offset displacement X0 of the valve element.
Beyond the cracking point in a current-displacement relationship map (i.e., for current greater than I0, and valve element displacement larger than X0), typically, a linear relationship is assumed between the applied current and the valve element displacement, albeit some non-linearity may exist. The linear relationship is typically characterized by a factor K relating current to displacement, such as, K=(X−X0)/(I−I0). In other words, the factor K represents a slope of a displacement-current curve. When an operator applies a command current I, the displacement X of the valve element may be calculated if K is known in addition to the known I0 and X0. The relationship between fluid flow rate Q through the valve and displacement X of the valve element is typically known. Therefore, the flow rate Q may be calculated if displacement X is known. Conversely, if a certain flow rate Q is needed to be supplied to a hydraulic component through the valve, from the known relationship between fluid flow rate Q and displacement X, the displacement X corresponding to the needed flow rate Q may be calculated. Then from the linear relationship between X and I characterized by the factor K=(X−X0)/(I−I0), the level of current I needed to produce valve element displacement X can be calculated. Therefore, it is important to have a relatively accurate estimate of the factor K in order to accurately characterize the displacement-current relationship of a hydraulic valve, and to accurately control hydraulic components through the valve.
Conventionally, the linear relationship characterized by the factor K may be determined, for example, through a laboratory calibration before installation of the valve for hydraulic control. This determined value of the factor K is treated as a nominal value of the factor K, and is assumed to be constant throughout the service life of the valve. In many cases, only one or a few valves representative of many valves of a same type are calibrated in the laboratory to obtain the nominal value of the factor K, which is then used for all remaining valves of the same type. However, due to various reasons such as variations among the solenoid characteristics of valves, valve spool manufacturing errors, and in-service wear of critical valve components, the actual value of the factor K may vary from one valve to another, and over time. Therefore, the nominal value of the factor K may not accurately reflect the actual current-displacement relationship of a valve. Using the nominal value of the factor K instead of the factor's actual value may cause significant error in the operation of the valve element and in the control of the hydraulic system. For example, when a large amount of current I is applied to the valve, even small difference between the actual value and the nominal value of the factor K may cause large variation in valve displacement X, which in turn may allow an improper amount of fluid to flow through the valve, and subsequently be delivered to other hydraulic components, such as a cylinder. Supplying an improper amount of fluid to the hydraulic components may cause controllability issues, and may adversely impact the components, which may ultimately result in malfunctioning or damage of the components and the hydraulic system.
Therefore, in applications, the use of an assumed constant value for the factor K rather than the actual value may result in large performance variation and may bring about issues such as elevated pump pressure, biased flow distribution, and malfunction of the hydraulic system where valves are employed. Although problematic valves may be disassembled from the machine where they are installed and sent to a laboratory for re-calibration, this process may require a significant amount of machine down time and may be costly in terms of labor. As a result, problematic valves, rather than being re-calibrated, may simply be discarded and replaced by new valves. On-machine valve calibration could reduce machine down time, save costs, and extend the service life of valves.
A method and system to calibrate hydraulic valves of an electrohydraulic system is described in U.S. Pat. No. 5,623,093 (the '093 patent) issued to Schenkel et al. on Apr. 22, 1997. The method and system determine two operating points associated with two positions of a hydraulic cylinder corresponding to magnitude of two electrical signals applied to an electrical valve. Then the two operating points are used to construct a basic valve curve representing the electrohydraulic characteristics of a particular machine, where interpolation may be performed to determine the electrical valve signal magnitude for any operating point between the two operating points.
While the '093 patent may provide for on-machine calibration of an electrohydraulic system, the method and system disclosed in the '093 patent require use of a joystick and position sensors for sensing the positions of the joystick. However, in some hydraulic system, a joystick and position sensors may not be available for calibrating valves. Therefore, the on-machine calibration method and system disclosed in the '093 patent may not be applicable to some hydraulic systems.
The method and system of the present disclosure are directed toward improvements in the existing technology.